U.S. Geological Survey
Open-File Report 98-462
Version 1.0

Sulfur Dioxide Emission Rates of Kīlauea Volcano, Hawai‘i, 1979-1997

1998

This report is preliminary and has not been reviewed for
conformity with U.S. Geological Survey editorial standards or with
the North American Stratigraphic Code. Any use of trade, product, or
firm names is for descriptive purposes only and does not imply
endorsement by the U.S. Government.

Sulfur dioxide (SO2) emission rates from Kilauea
Volcano were first measured by Stoiber and Malone (1975) and have
been measured on a regular basis since 1979 (Casadevall and others,
1987; Greenland and others, 1985; Elias and others, 1993; Elias and
Sutton, 1996). The purpose of this report is to present a compilation
of Kilauea SO2 emission rate data from 1979 through 1997
with ancillary meteorological data (wind speed and wind direction).
We have included measurements previously reported by Casadevall and
others (1987) for completeness and to improve the usefulness of this
current database compilation.

Kilauea releases SO2 gas predominantly from its summit
caldera and rift zones (fig. 1). From 1979 through 1982, vehicle-based COSPEC measurements
made within the summit caldera were adequate to quantify most of the
SO2 emitted from the volcano. Beginning in 1983, the focus
of SO2 release shifted from the summit to the east rift
zone (ERZ) eruption site at Pu‘u ‘Ō‘ō and, later, Kupaianaha. Since
1984, the Kilauea gas measurement effort has been augmented with
intermittent airborne and tripod-based surveys made near the ERZ
eruption site. In addition, beginning in 1992 vehicle-based
measurements have been made along a section of Chain of Craters Road
approximately 9 km downwind of the eruption site. These several types
of COSPEC measurements continue to the present.

Figure 1. Summit and rift zones of Kilauea Volcano. From
1979 to 1997, most of the SO2 at Kilauea was released from the summit
and east rift zone. Vehicle-based COSPEC measurements were made in
the summit caldera and along Chain of Craters Road during trade wind
conditions, to quantify summit and east rift zone emissions
respectively. Intermittent airborne and tripod-based surveys made
near Pu‘u ‘Ō‘ō, augmented the vehicle-based studies.

ACKNOWLEDGEMENTS

Numerous individuals including volunteers and staff of the
Hawaiian Volcano Observatory
(HVO) contributed to these approximately 1100 days of measurements,
which include more than 5000 plume traverses and span 19 years of
volcano monitoring. These measurements were funded under the Volcano
Hazards, Global Change, and Geothermal Research Programs.

METHODS AND
UNCERTAINTIES

Instrumentation

From 1979 through 1984, SO2 measurements at Kilauea
were made with a COSPEC IV correlation spectrometer fitted with a
standard SO2 correlation mask disc assembly and
calibration cells of 89 ppm and 358 ppm. During eruptive periods in
1979 and 1982-1984, the burden of SO2 in the eruption
plumes was high enough to saturate the instrument. In order to
measure higher burdens of SO2, a high concentration disc
assembly and higher concentration calibration cells (402 ppm and 1447
ppm) were installed in the instrument in 1984. The COSPEC was fitted
with a Maxwellian telescope until June 1994, when a Cassegrain
telescope was installed. Comparison measurements were made during
daily data collection for several weeks during a period of relatively
steady emission rates. These measurements suggest good agreement
between the data collected with the two different types of
telescopes.

Summit vehicle-based data

The emission rate measurements at the summit of Kilauea reported
here were made by vehicle-based COSPEC traverses within the summit
caldera along Crater Rim Drive, as described by Casadevall and others
(1987). The road traverse path used during typical trade-wind
conditions is shown in figure 2. The data for 1979 through 1997 are
shown in table 1a, table 1b and figure
3. Note that wind vector and standard deviation data are not
reported for 1979-1981, as these parameters were unavailable.
Casadevall and others (1987) note that data from September 1980
through October 1981 are considered to be of poor quality owing to
the infrequency of data collection and inexperience of operators.
With this caveat, the rest of the summit data from 1979 through 1997
are judged to be of good quality.

Figure 2. Aerial view showing the summit COSPEC traverse path at Kilauea
Caldera. The most notable sources of SO2 in the summit region are fumaroles in and
near Halema`uma`u Crater, and solfataras within the caldera. During prevailing trade-wind
conditions, the plume is blown across Crater Rim Drive and thus can be measured by driving
underneath with a vehicle-mounted COSPEC.

Figure 3. Kilauea summit SO2 emission rates in
metric tons per day from June 1979 through 1997. Summit emissions
increased with the onset of the 1983 Pu‘u ‘Ō‘ō eruption. Another
increase is seen in early 1987, and summit emissions have declined
from an average of 410 mt/d for February 1987 to 140 mt/d for
February 1997.

Wind velocities for the summit COSPEC measurements were made 1 m
above the ground near HVO before and after each set of COSPEC
traverses, using a 5-minute average determined from a wind-counter.
Our experiments have shown that wind speeds measured 3 m above the
ground near this site are 20% higher than the 1 m measurements but
are from nearly the same direction. Since calculated emission rates
are directly proportional to wind speed, emission rates determined
using 1 m wind speeds produce data that are likely to be minimum
values. To improve the accuracy of our emission rate measurements, in
January 1998 we began calculating SO2 emission rates using
wind data measured 3 m above the ground, at a site 30 m west of the
edge of the summit caldera (fig. 1). We believe that data from this
site give a more reasonable representation of wind conditions at the
elevation of the plume than the 1 m winds, and will contribute to a
more accurate estimate of the SO2 released from the summit
of Kilauea. For accuracy and continuity, therefore, the vehicle-based
summit emission rate data presented in this report from 1979 through
1997 have been adjusted by +20% to reflect the wind speed correction.
The reported wind speeds are also corrected values - they are
increased by 20% from the measured values. Based on our wind
characterization experiments, we estimate the uncertainty in summit
wind speed measurements to be 10-20%.

East rift vehicle-based data

For wind conditions with speeds greater than 5 m/s and directions
between 25 o and 40o east, gases from Pu‘u ‘Ō‘ō
typically form a compact plume that crosses Chain of Craters Road
above the 180o turn at Holei Pali (fig. 1). Since 1992,
vehicle-based COSPEC data obtained under these conditions have
typically provided the best integrated estimate of Kilauea's ERZ
SO2 release (table 2 and
fig. 4). These measurements
are made using the same procedure as for those made at the summit.
Through mid-1994, wind velocities were determined using a variety of
techniques, including: 5-minute wind-counter measurements at 1 and 4
m above the ground made before and after COSPEC traverses; tracking
pilot balloons; and continuous, 5-minute wind-counter measurements
made at 4 m during a day's COSPEC traverses. In 1994, we installed a
continuous wind monitor 3.5 m above the ground, approximately 2.5 km
above the 180o turn on Chain of Craters Road, to provide
improved and contemporaneous wind speed determinations. East rift
emission rates measured along Chain of Craters Road were reported
earlier by Andres and others (1989). These measurements were made
using wind vector data from twice-daily National Weather Service
balloon releases at the Hilo airport, 12 m above sea level and 45 km
northeast of the measurement site (R. Andres, personal communication,
1997). We believe that the continuous wind monitor may provide a more
accurate estimate of wind velocity for the Chain of Craters Road
measurements, because the east rift plume is frequently close to the
ground as it crosses Chain of Craters Road. The uncertainty in wind
speed measurements from Chain of Craters Road is estimated to be
10-20%

Figure 4. Averaged SO2 emissions from Kilauea's
east rift zone as measured by vehicle-based COSPEC along Chain of
Craters Road, 1992 through 1997. The black vertical bars represent
the standard deviation of all traverses on a single day. These
measurements have typically provided the best integrated estimate of
Kilauea's ERZ SO2 release.

East rift tripod-based data

From 1984 through 1991, tripod-based measurements were made from
Pu‘u Kamoamoa (KMM), approximately 1 km uprift of Pu‘u ‘Ō‘ō
(fig. 5). Data are shown in
table 3 and figure 6. Wind
velocities were determined concurrently with the COSPEC measurements
using a 5-minute average from a wind-counter located 1 m above the
ground. A constant distance to the plume of 0.7 km was used to reduce
these data, regardless of wind vector. The measurements made from KMM
span a time period that includes episodic fountaining at Pu‘u ‘Ō‘ō
(1983-1986) and continuous quiescent activity at the Kupaianaha vent
(1986-1992). These eruptive episodes have been described by Wolfe and
others (1988) and Heliker and others (1987,1991). Tripod-based
SO2 data reported for 1984-1985 (fig. 6, filled boxes)
generally characterize degassing at Pu‘u ‘Ō‘ō during the repose
periods between fountaining events. However, on July 28, 1984, COSPEC
data were collected during the eruptive episode 23 fountaining. At
other times, measurements were made just after fountaining events or
during other non-fountaining activity during this time period; these
data are labeled on figure 6.

Figure 5. Aerial view of tripod-based COSPEC measurement
sites on the east rift zone, 1984 to 1997. Through 1991, tripod-based
measurements were made from Pu‘u Kamoamoa (KMM), approximately 1 km
uprift of Pu‘u ‘Ō‘ō. From 1992 through 1995, changes in the eruptive
vent, degassing character, and local topography forced us to seek
alternate instrument sites. Sites STC, Vince, and N. of PU, all in
use since 1995, permitted emission-rate measurements under a variety
of wind and eruptive conditions.

Figure 6. Averaged SO2 emissions from Kilauea's east rift
zone as measured by tripod-based COSPEC from 1984 through 1997. The
black vertical bars represent the standard deviation of all
measurements on a single day. Emission rates for 1984-1985 (filled
boxes) generally characterize degassing at Pu‘u ‘Ō‘ō during the
repose periods between fountaining events. However, eruption episode
(EE) 23 data were collected during a fountaining event. Eruption
episode 28 data were collected several hours after the end of a
fountaining event, and eruption episode 19 and 35A measurements were
made during other non-fountaining activity. Measurements of Pu‘u ‘Ō‘ō
emissions made during eruption episode 48 (filled circles) were
discontinued in late 1991. The hollow circles, stars, and filled
triangles denote emission-rate data from various instrument sites
used during 1995-1997 and span periods of eruptive pause, eruptive
surge, and varying levels of vent activity.

Chartier and others (1988) describe the general data collection
procedure for tripod-based measurements and report a detailed record
of emissions during a 1985 repose period.

In 1986, the eruptive activity shifted to the Kupaianaha vent.
During this time, Pu‘u ‘Ō‘ō continued to act as a chimney, venting
gases as the magma traveled farther down rift to Kupaianaha.
Degassing from Pu‘u ‘Ō‘ō formed a consolidated plume which commonly
lofted above the ground—a plume geometry favorable for making
tripod-based COSPEC measurements (fig. 6, filled circles).

From 1992 through 1995 a series of vents, fissures, collapses, and
a broad shield developed on the flanks of Pu‘u ‘Ō‘ō (Heliker and
others, 1998.) The narrow conduit inside the cone, which earlier
focused the gas release, eventually enlarged to form a 300-m diameter
crater. KMM was covered beneath tens of meters of lava, and acute
changes in local topography occurred. The enlargement of Pu‘u ‘Ō‘ō
crater, and an increase in the number of east rift degassing sites,
contributed to a less well defined plume. These factors, along with
changes in local topography, forced us to seek other sites when
tripod-based measurements were resumed in 1995 (fig. 5). Sites
indicated as STC, Vince, and N. of PU permitted emission-rate
measurements under a variety of wind and eruptive conditions. The
data for these measurements are shown in table 4 and figure 6 and
span periods of eruptive pause, eruptive surge, and varying levels of
vent activity.

Results of several other tripod surveys are shown in table 4,
including emission-rate measurements made on individual skylights on
the upper tube system just south of Pu‘u ‘Ō‘ō and also from vents on
the flank of Pu‘u ‘Ō‘ō. Two sets of measurements are included that
were made on the eruptive episode 54 vent in Napau Crater (fig. 1,
table 4). For these measurements, wind velocity was determined with a
wind-counter mounted on a pole 4 m above the ground using 5-minute
averaging. Observed readings showed reasonable agreement when
compared with continuous wind monitors located nearby. A variety of
methods was used to determine the distance to, and width of, the
plume, including paced distances, recent air photos taken during
known wind conditions, and helicopter-borne GPS.

East rift and summit airborne
measurements

Airborne measurements of SO2 emission rates at Pu‘u ‘Ō‘ō (Casadevall and others, 1987) and at Kilauea's summit caldera
(Greenland and others, 1985) were made during 1983 and 1984 and are
presented in table 5. Plumes from eight of the Pu‘u ‘Ō‘ō eruptive
episodes and two of the inter-eruptive periods were quantified using
the airborne platform. During Pu‘u ‘Ō‘ō fountaining, enormous
SO2 burdens were measured, while during inter-episode
periods emissions declined significantly. Casadevall and others
(1987) note that the concentration of SO2 exceeded that
measurable by the COSPEC, thereby making the east rift data from 1983
through 1984 minimum emission rates. The inter-episode airborne
measurements are generally consistent with the inter-episode
tripod-based measurements. Likewise, summit airborne measurements
agree well with the vehicle-based summit measurements made on the
same day.

Airborne SO2 measurements of the east rift eruption
plume were made again on September 19, 1995, after a lapse of 11
years. The changes in vent geometry, along with the advent of
multiple degassing sources that began in 1992, gave rise to a broad
plume whose bottom was at ground level (McGee and Gerlach, 1998).
Thus, the emission rate reported for this date in table 5 is likely a
minimum constraint, as the COSPEC traverses were flown 500' above
ground level.

Technique comparisons

Vehicle- and tripod-based ERZ emission-rate measurements were made
in 1995-1997. COSPEC measurements along Chain of Craters Road yielded
emission rates consistently higher than those from near-vent
tripod-based measurements (fig.
7). Andres and others (1989) also found a difference in vehicle
and tripod-based measurements; vehicle-based measurements along Chain
of Craters Road were 1.3-2 times greater than tripod-based data. We
speculate that a combination of factors may contribute to this
discrepancy, including multiple degassing sources, plume thickness
and geometry, distance from the measurement site to the plume, and
instrument configuration,. Although the tripod-based measurements
provide important information on vent activity, these emission rates
should be interpreted with care. Vehicle-based COSPEC data collected
along Chain of Craters Road provide the best estimate of Kilauea's
integrated ERZ SO2 release from 1992-1997 and are
supported by the 1995 airborne measurements. Airborne measurements
also provide a better estimate of total SO2 released from
the east rift zone during eruptive fountaining episodes than do
tripod-based measurements.

Data quality codes are presented with the Chain of Craters Road,
vehicle-based data and 1995-1997 tripod-based data in table 2 and table 4.
For the vehicle data, the following convention is used:

A - Best quality data - usually with strong, steady wind
conditions, and a compact, consistent plume shape.

B - Good quality data - usually with moderately consistent plume
shape and location of plume on road. Collected under moderately
strong, uniform winds, with a good constraint on wind speed and
direction.

C - Acceptable data - may have inconsistent plume location and
shape. Wind speed and direction may be variable or poorly
constrained. Some runs may measure a partial plume, representing
minimum constraints on emission rates. Measurements with instrument
inconsistencies are included in this category.

For the 1995-1997 tripod-based data, the following convention is
used:

A - Best quality data - distance to plume less than 1 kilometer,
and good constraint on measurement of distance; steady wind direction
resulting in COSPEC angle consistently orthogonal to plume.

C - Acceptable data - used indirect method to estimate distance to
plume; measurements may have been made under less consistent wind
conditions. Local terrain and increased distance from the plume may
compromise measurement geometry.

The wind direction and wind speed can serve as a rough indicator
of the data quality for vehicle-based measurements at Kilauea's
summit. Conditions with wind speeds greater than 3 m/s and directions
between 340o and 60 o generally indicate a
sufficiently consolidated plume to be accurately quantified.